i presume that your car is a hatchback, if that is the case it will be the tailgate struts that have lost there power and will need renewing.its a simple job as the metal plate holding the struts just slide of at both ends and then remove the old strut, see here for new struts.

It's worth noting what I was told when I asked much the same question last year. Basically, that the gas struts are less efficient in winter as the gas inside is more dense, so doesn't push the strut out as far as it should. This results in me cracking my head on the boot lid more often than I'd like in winter, as it hadn't gone up as high as I thought it was going to. Once a week seems to be the average rate for this happening.

But yeah, if it's dropping all the way down again, the struts need replaced.